xenguest-manager: Wait longer for /dev/xvdaX creation

Implement a wait block of 20s with exit condition for checking
/dev/xvdaX creation.
For example, in cases where the rootfs is mounted via NFS, a longer
time is needed until the xvda device appears in the system.

Change-Id: I44cbcf2a43aeb476eae92d5b6d2cfd683e4bdf21
Issue-Id: SCM-929
Signed-off-by: Diego Sueiro <diego.sueiro@arm.com>
Reviewed-by: Bertrand Marquis <bertrand.marquis@arm.com>
Signed-off-by: Jon Mason <jon.mason@arm.com>

meta-arm-autonomy/recipes-extended/xenguest/files/xenguest-manager

@@ -234,10 +234,17 @@ function xenguest_disk_init()
                 exit 1
             fi
 
-            # Sometimes it takes time to see the device
-            if [ ! -b /dev/xvda${part} ]; then
-                sleep 1
-            fi
+
+            # Loop for 20s to wait until /dev/xvdaX appears
+            i=0
+            while [ ! -b /dev/xvda${part} ]; do
+                ((i++))
+                if [[ "$i" == '40' ]]; then
+                    break;
+                fi
+                sleep 0.5
+            done
+
             if [ ! -b /dev/xvda${part} ]; then
                 echo "${PREF} Partition ${part} creation error"
                 xenguest_detach_disk